Drug development programs rely on in vitro screening assays and subsequent testing in appropriate animal models to evaluate drug candidates prior to conducting clinical trials using human subjects. Screening methods currently used are generally difficult to scale up to provide the high throughput screening necessary to test the numerous candidate compounds generated by traditional and computational means. Moreover, studies involving cell culture systems and animal model responses frequently don't accurately predict the responses and side effects observed during human clinical trials.
Conventional methods for assessing the effects of various agents or physiological activities on biological materials, in both in vitro and in vivo systems, generally are not highly sensitive or informative. For example, assessment of the effect of a physiological agent, such as a drug, on a population of cells or tissue grown in culture, conventionally provides information relating to the effect of the agent on the cell or tissue population only at specified points in time. Additionally, current assessment techniques generally provide information relating to a single or a small number of parameters. Candidate agents are systematically tested for cytotoxicity, which may be determined as a function of concentration. A population of cells is treated and, at one or several time points following treatment, cell survival is measured. Cytotoxicity assays generally do not provide any information relating to the cause(s) or time course of cell death.
Similarly, agents are frequently evaluated based on their physiological effects, for example, on a particular metabolic function or metabolite. An agent is administered to a population of cells or a tissue sample, and the metabolic function or metabolite of interest is assayed to assess the effect of the agent. This type of assay provides useful information, but it does not provide information relating to the mechanism of action, the effect on other metabolites or metabolic functions, the time course of the physiological effect, general cell or tissue health, or the like.
Optical techniques have been developed and used for several applications. Light scattering has been used in the past to provide measurements of osmotic water permeability in suspensions of osmotically responsive vesicles and small cells. A. S. Verkman, "Optical Methods to Measure Membrane Transport Processes," J. Membrane Biol. 148:99-110, 1995. Another study reported a method for the optical measurement of osmotic water transport in cultured cells. M. Echevarria, A. S. Verkman, "Optical Measurement of Osmotic Water Transport in Cultured Cells: Role of Glucose Transporters," J. Gen. Physiol. 99:573-589, 1992.
Optical techniques for observing nerve activity and neuronal tissue are well-established. Hill and Keynes observed that the nerve from the walking leg of the shore crab normally has a whitish opacity caused by light scattering, and that opacity changes evoked by electrical stimulation of that nerve were measurable. Hill, D. K. and Keynes, R. D., "Opacity Changes in Stimulated Nerve," J. Physiol. 108:278-281, 1949. Since the publication of those results, experiments designed to learn more about the physiological mechanisms underlying the correlation between optical and electrical properties of neuronal tissue and to develop improved techniques for detecting and recording activity-evoked optical changes have been ongoing.
Intrinsic changes in optical properties of cortical tissue have been assessed by reflection measurements of tissue in response to electrical or metabolic activity. Grinvald, A., et al., "Functional Architecture of Cortex Revealed by Optical Imaging of Intrinsic Signals," Nature 324:361-364, 1986; Grinvald, et al., "Optical Imaging of Neuronal Activity, Physiological Reviews, Vol. 68, No. 4, October 1988. Grinvald and his colleagues reported that some slow signals from hippocampal slices could be imaged using a CCD camera without signal averaging.
A CCD camera was used to detect intrinsic signals in a monkey model. Ts'o, D. Y., et al., "Functional Organization of Primate Visual Cortex Revealed by High Resolution Optical Imaging," Science 249:417-420, 1990. The technique employed by Ts'o et al. would not be practical for human clinical use, since imaging of intrinsic signals was achieved by implanting a stainless steel optical chamber in the skull of a monkey and contacting the cortical tissue with an optical oil. Furthermore, in order to achieve sufficient signal to noise ratios, Ts'o, et al., had to average images over periods of time greater than 30 minutes per image.
The mechanisms responsible for intrinsic signals are not well understood. Possible sources of intrinsic signals include dilation of small blood vessels, neuronal activity-dependent release of potassium, and swelling of neurons and/or glial cells caused, for example, by ion fluxes or osmotic activity. Light having a wavelength in the range of 500 to 700 nm may also be reflected differently between active and quiescent tissue due to increased blood flow into regions of higher neuronal activity. Yet another factor which may contribute to intrinsic signals is a change in the ratio of oxyhemoglobin and deoxyhemoglobin in blood.
U.S. Pat. No. 5,215,095 discloses methods and apparatus for real time imaging of functional activity in cortical areas of a mammalian brain using intrinsic signals. A cortical area is illuminated, light reflected from the cortical area is detected, and digitized images of detected light are acquired and analyzed by subtractively combining at least two image frames to provide a difference image. Allowed U.S. patent application Ser. No. 08/474,754 discloses similar optical methods and apparatus for optical detection of neuronal tissue and activity.
U.S. Pat. No. 5,438,989 discloses a method for imaging margins, grade and dimensions of solid tumor tissue by illuminating the area of interest with high intensity electromagnetic radiation containing a wavelength absorbed by a contrast agent, obtaining a background video image of the area of interest, administering a contrast agent, and obtaining subsequent video images that, when compared with the background image, identify the solid tumor tissue as an area of changed absorption. U.S. Pat. No. 5,699,798 discloses methods and apparatus for optically distinguishing between tumor and non-tumor tissue, and imaging margins and dimensions of tumors during surgical or diagnostic procedures.
U.S. Pat. No. 5,465,718 discloses a method for imaging tumor tissue adjacent to nerve tissue to aid in selective resection of tumor tissue using stimulation of a nerve with an appropriate paradigm activate the nerve, permitting imaging of the active nerve. The '718 patent also discloses methods for imaging of cortical functional areas and dysfunctional areas, methods for visualizing intrinsic signals, and methods for enhancing the sensitivity and contrast of images. U.S. Pat. No. 5,845,639 discloses optical imaging methods and apparatus for detecting differences in blood flow rates and flow changes, as well as cortical areas of neuronal inhibition.
U.S. Pat. No. 5,902,732 discloses methods for screening drug candidate compounds for anti-epileptic activity using glial cells in culture by osomotically shocking glial cells, introducing a drug candidate, and assessing whether the drug candidate is capable of abating changes in glial cell swelling. This patent also discloses a method for screening drug candidate compounds for activity to prevent or treat symptoms of Alzheimer's disease, or to prevent CNS damage resulting from ischemia, by adding a sensitization agent capable of inducing apoptosis and an osmotic stressing agent to CNS cells, adding the drug candidate, and assessing whether the drug candidate is capable of abating cell swelling. A method for determining the viability and health of living cells inside polymeric tissue implants is also disclosed, involving measuring dimensions of living cells inside the polymeric matrix, osmotically shocking the cells, and then assessing changes in cell swelling. Assessment of cell swelling activity is achieved by measuring intrinsic optical signals using an optical imaging screening apparatus.